Apparatus similar to the apparatus of the present invention is described in EP-A-0 346 275. In this apparatus, each braking ball is mounted in a recess of an axial extension of the plug guide which is larger than its diameter and a fixed wedge drives the ball towards the plunger when the latter, propelled forwardly, entrains it forwardly in its recess.
This arrangement was aimed at reducing the drawback of a plunger retaining ball mounted in a recess of an axial extension substantially equal to its diameter and subject to the permanent radial action of a spring, even during rearward and therefore more difficult return of the plunger, as taught by DE-C-1 058 950.